The present invention is related to control systems and more specifically, to control systems for controlling the speed of variable speed steam turbines.
Variable speed steam turbines typically employ a feedback loop to control the shaft speed of the steam turbine to an approximation of a commanded speed. One element in such control systems includes a speed signal generator producing a signal for comparison with the commanded speed.
For high reliability, two or more independent channels may be provided for generation of independent speed feedback signals.
In a digital speed signal generator, it has been common to employ a toothed wheel coupled to the shaft whose speed is to be measured. The toothed wheel generates an electrical signal as the teeth pass in the vicinity of a sensor. The resulting pulse information is first converted into an analog voltage with a frequency-to-voltage converter followed by an analog-to-digital conversion. This technique has the disadvantage of complexity in implementation and of having its resolution limited by the converters. In addition, such a system requires calibration.
Reliability in turbine control has been improved by employing redundant controllers, each capable of providing a control signal to control the turbine. A problem in the use of such redundant controllers is to detect failure of a controller and to transfer control to one which has not failed. One technique previously used employs three or more controllers, each receiving identical inputs and an added hardware or software system to select the controller to be used on the basis of voting. Alternatively, the outputs of multiple controllers may be combined in such a fashion that a failure in one of them which places its output out of range control will be ignored.
When two or more controllers are employed operating in parallel to produce corresponding signals, only one of which is used, when a fault is detected in the active controller, control is transferred to the previously inactive controller. Over time, the output of the previously inactive controller may drift slightly so that the active and inactive control signals, even though based on substantially identical inputs, have diverged. When switchover of source of control signal is accomplished in this circumstance, a bump or discontinuity in the control signal applied to the steam turbine is experienced. Such discontinuity can cause a rapid pulse in the acceleration of the steam turbine resulting in a change of speed which takes a finite time to overcome using the feedback speed signal.